Reference to related application, assigned to the Assignee of the present invention, the disclosure of which is hereby incorporated by reference.
U.S. Ser. No. 08/176,701 filed Jan. 3, 1994, B. TAMAYO LOPEZ et al. claiming priority of Spanish application P 9202675.
Reference to related publications:
The "Flux-gate magnetometer" by F. Primdahl, the entire contents of which is incorporated herein by reference, published in the J. Phys. E: Sci. Instrum., vol. 12, 1979, pages 241 et. seq.
"Switch capacitor circuits" by Philip E. Allen & Edgar Sanchez-Sinenchio, Van Nostrand Reinhold Company Inc., 135 West 50th Street, N.Y. 10020 USA.
Databook Volume 1, 1989, National Semiconductor, "Data Acquisition Linear Devices, pages 1-152 to 1-173, dev.: MF8, 4th-Order Switched Capacitor Bandpass Filter; and
Databook Volume 2, Texas Instruments, "Linear Circuits" Data Acquisition and Conversion, pages 2-139 to 2-153, dev.: TLC10/MF10A, TLC20/MF10C, Universal Dual Switched-Capacitor Filter.
1. Field of the Invention
This invention concerns a method for relaxing internal stress produced in the core of the a high sensitivity magnetometric sensor during the operation of a coil winding, where the primary or drive winding is wound directly on the core of an amorphous ferromagnetic material covered only by a strip of insulating tape.
This method is applicable to the process of manufacturing high sensitivity magnetometric sensor core heads of the flux-gate type.
2. Background
The use of magnetometric sensors based on the flux-gate principle has been known from the 1950s and, since then, although the basic principle has not changed, great effort has been dedicated to perfecting these devices, especially with respect to noise reduction and boosting sensitivity.
In an article entitled, "The Fluxgate Magnetometer" by F. Primdahl, the entire contents of which is incorporated herein by reference, published in the J. Phys. E: Sci. Instrum., vol. 12, 1979, the fluxgate principle is explained in detail. The basic flux-gate principle is outlined on page 241 of said Primdahl articles.
A sensor using the fluxgate principle as described on page 241 of the Primdahl article comprises a core formed of a magnetic material which has a pick-up coil mounted thereon.
The earth's magnetic field acting in a direction parallel to the core axis will cause a magnetic flux to appear in the core and will also cause an induced voltage to appear across the pick-up coil. Any changes in the permeability of the core will cause the flux in the core to change and the induced voltage in the pick-up coil to change. This change in flux resulting from changes in the core permeability, will induce the voltage to appear across the pick-up coil mounted on the core. The magnitude of change in core permeability depends on the core material used and the geometric shape of the core material.
This basic flux-gate action is based on a time variation of the core permeability. When a magnetic material is saturated, its permeability to further magnetization decreases. The well known hysteresis curves for magnetic materials reflect the fact that the magnetization of the core decreases when the core is saturated. If an alternating voltage or current is applied to the saturated core, then an alternating voltage or current will appear across the pick-up coil.
In the article "Sensor noise in low-level flux-gate magnetometers" by D. C. Scouten, published in the IEEE Transactions on Magnetics review, volume MAG-8 No.2, June 1972, page 228, it is shown how noise is produced through non-alignments in the structure of the core material and that their effect is equivalent to independent magnetic moments that occur inside the core. These non-alignments can be produced, and in fact are produced, by mechanical stress, including direct winding of a coil on the core, during the manufacturing process of the sensor head.
In the article "An Evaluation of the Noise Performance of Fe, Co, Si and B amorphous alloys in ring-core fluxgate magnetometers" by B. B. Narod et al., published by CAN. J. PHYS., volume 63, 1985, on page 1472 it is indicated how major improvements can be achieved in relation to noise characteristics by means of sophisticated methods of core annealing.
At present, the usual, or standard procedure is annealing the unwound core in an oven at a temperature above the Curie temperature, at which the internal stress relax and the nonalignments that produce disoriented magnetic couples disappear, thereby significantly reducing the noise produced by the core in question. The high temperatures normally reached prevent annealing of the core once wound, since the wire or insulation of the enveloping winding could be damaged, unless a special, and much more expensive wire and insulation is employed.
For this reason it has been proposed to wind the coils on coil forms or cylinders that can stand the winding forces and the following manufacturing processes, instead of these having to be supported directly by the core itself.
This means that more electromagnetic energy would be required in order to saturate the core which increases the power consumption.